4 Methanol Production

Methanol was first produced as a by-product in the manufacture of
charcoal through the destructive distillation of wood, with yields of 12-24
lifers per ton of wood. Most methanol today is produced from natural gas. In
principle, many carbon-containing materials may be substituted for natural gas
as starting materials. These include (in addition to wood) coal, lignite, and
even municipal wastes. Each of these raw materials, however, must first be
converted to syngas; for this step, each alternative feedstock requires process
modifications that increase capital investment costs over those required for
natural gas.

Descriptions follow of the processes for converting natural gas
and wood to methanol.

METHANOL FROM NATURAL GAS

The production of methanol through the conversion of natural gas
to syngas is used in conventional methanol plants throughout the world.
Typically' natural gas-primarily methane-is catalytically reacted with steam and
carbon dioxide to yield hydrogen and carbon monoxide in a ratio of 2:1.

The production of methanol through the conversion of wood to
syngas is being examined in several countries. In terms of converting carbon to
methanol, wood is inherently less efficient than natural gas. The initial
gasification step in producing syngas from wood yields a mixture of CO and H2
deficient in hydrogen; to bring the ratio of H2 to CO to 2:1, part of the CO is
reacted with steam to yield additional hydrogen:

This carbon dioxide is then removed from the process stream and
discarded. Approximately 50 percent of the carbon in the wood entering the
process is nonproductively released to the atmosphere. This also means that 50
percent of all the forestry and transport activities to provide the wood to the
plant are wasted. About 2.25 kg of natural gas are required to produce 4 lifers
of methanol as opposed to about 9 kg of dry wood.

Figure 25 shows the sequence of operation suggested by Hokanson
and Rowell for the production of methanol from wood.

Suggested Steps for Wood Conversion Gasification

In the first step, wood is charged at the top of the reactor and
ash discharged from the bottom. Air and steam are charged near the base of the
reactor.

The sequence of reactions occurring in wood gasification are as
follows:

The raw gas is then purified to remove all but hydrogen and carbon
monoxide. This mix is reacted to convert part of the CO to H2 So that
the final mixture contains a 2:1 ratio of H2 to CO. In this
conversion, additional CO2 is formed and must be removed before
methanol synthesis.

In detail, the raw gas from the reactor passes through a scrubber
(2) cooling the gas to about 32° C and removing tars and acid.

The gas is then compressed to about 100 psig (3) and treated in
two stages to remove carbon dioxide. In the first stage (4), a hot potassium
carbonate solution reduces the CO2 content to about 300 ppm. In the
second stage (5), monoethanolamine (MEA) is used to reduce the CO2
content to about 50 ppm.

The gas then passes through a cryogenic system (6), which removes
the residual CO2 and water vapor, plus methane and other hydrocarbons
and finally nitrogen.

The purified gas is a mixture of hydrogen (approximately 44
percent) and carbon monoxide (approximately 56 percent). It requires further
processing to provide the 2:1 ratio of H2 to CO needed to produce
methanol.

Following cryogenic purification, the gas is compressed (7) to 400
psig for shift conversion. Part of the CO reacts with water vapor in the
presence of an iron catalyst to form additional hydrogen (8) so that the exit
gas contains a 2:1 ratio of H2 to CO.

Since the shift reaction (equation 3) also produces
CO2, it is necessary to rescrub the gas with a potassium carbonate
absorption system (9).

The syngas is then compressed (10) to 2,000-4,000 psig and passed
into the methanol synthesis reactor ( 1 1). In the reactor, approximately 95
percent of the gas is converted to methanol over a zinc-chromium catalyst. The
unreacted gasses are separated and recycled and the methanol purified by
distillation (12).

Processing Alternatives

Other investigators have examined alternative methods in the
conversion of wood to methanol.

Earl describes a process in which oxygen instead of air is used in
the gasification step. This requires an air separation plant at the beginning of
the process, but eliminates the cryogenic step described by Hokanson and Rowell.

Mundo and Wehner suggest that locating a wood-to-methanol plant at
a hydroelectric site would have several advantages. Electrolytic oxygen could be
used in gasification and electrolytic hydrogen could be used to adjust the H2:CO
ratio for syngas production instead of shifting carbon monoxide. Location at a
dam might also allow less expensive water transport for wood.

Osler, reporting on methanol production options in Canada,
compares simple gasification of wood with hybrid systems that also utilize
hydrogen or methane. To produce 1 ton of methanol, either 2.33 tons of wood or
0.9 tons of wood with 0.12 tons of hydrogen or 0.4 tons of wood with 867 cubic
meters of natural gas would be required.

Arbo describes a process in which low-grade coal is briquetted
with cellulosic wastes and gasified as a possible first step for methanol
production.

Rooker details the requirements for methanol production from
Brazilian eucalyptus wood. For a 2,000-ton-per-day methanol facility, the total
plant investment would be $275 million.

Haider covers methanol production considerations, including
thermodynamics, catalysis, and kinetics, as well as a description of the low-,
medium-, and high-pressure processes for converting syngas to methanol.

Laboratory studies at Battelle-Northwest have defined conditions
and catalyst systems for the steam gasification of wood to synthesis gas. These
results have been used by Davy-McKee, Inc., to prepare a conceptual plant design
for methanol production from wood. Capital and operating costs for methanol
plants with annual capacities of about 2.5, 10, and 100 million gallons have
been projected and are shown in Table 10.

TABLE 10 Methanol Production Costsa

Methanol

Operational Characteristics

(in millions of gallons per year)

Production

Level

2.50

9.91

98.81

Wood capacity (dry tons per day)

50

200

2,000

Capital required (millions of dollars)

16.4

41.2

145.6

Annual operating costs (millions of dollars)

2.9

7.8

36.9

Methanol cost (dollars per gallon)

Utility financing

1.99

1.30

.55

Private financing

2.62

1.70

.69

a Based on wood at $20 per dry ton.

SOURCE: Batelle-Northwest, Richland, Washington, USA.

International Harvester is developing a small-scale methanol plant
that would be factory built in major modules and then transported to the
production site. Because of the savings made possible by producing identical
units in quantity in a factory, a package plant could produce methanol for the
same price as a plant 15 times its size built by on-site construction. Capital
costs of about $0.25 per annual lifer are projected for plants with capacities
of 25-30 million lifers per year. Such plants would utilize 100-150 dry tons of
wood per day.

The Energy Transition Corporation of Santa Fe, New Mexico, is
planning a peat-to-methanol project in North Carolina. A gasifier developed by
Koppers Co. and Babcock and Wilcox is expected to convert approximately 700,000
tons per year of peat to 250 million lifers of methanol.Sweden's Board for
Energy Source Development has spent $4.5 million to build and test a pilot plant
that can convert 12 dry tons per day of peat or wood (via syngas) to about
10,000 lifers of methanol.

REFERENCES AND SELECTED READINGS

Methanol Production

Anonymous. 1979. Brazil out to show methanol grows on trees.
Chemical Week March 14:38.

Mundo, KJ., and Wehner, H. 1979. Production of methanol From wood:
processes foresting and economics. Third International Symposium on Alcohol
Fuels Technology, May 28, 1979, Asilomar, California. Available from NTIS (Order
#CONF-790520).

Osler, C.F. 1979. Canadian Scenario for Methanol Fuel. Third
International Symposium on Alcohol Fuels Technology, May 28, 1979, Asilomar,
California. Available from NTIS (Order #CONF-790520).